This invention relates to lithography and more particularly to a lithographic method combining large area interference lithography with local area direct write focused laser spike annealing (FLaSk).
In order to advance lithographic capabilities for future applications, innovative strategies must be available in a scalable and cost-effective set of techniques. All lithographic techniques can be considered as either large or small area with respect to the features they pattern. While this is a rather simplistic classification scheme, it illustrates the dichotomy of lithographic strategies, namely: speed versus control. Both are necessary traits of any scalable process. One goal of alternative lithographic strategies is the fabrication of complex 3D architectures; most current 3D fabrication techniques rely on the stacking of multiple 2D structures. This places significant limitation on aspects such as features that cross multiple levels, the ability to have multiple material types in close proximity at the same height, and minimum resolution due to registry and stability considerations. In addition, with more fabricated layers, the number of fabrication steps rapidly increases. The ability to pattern 3D structures of controllable positioning, size, and symmetry across multiple length scales will enable a new generation of optical, acoustic, mechanical, plasmonic, electronic, and other devices. The variety of techniques for creating complex 3D assemblies is ever increasing, along with the variety of possible structures and the degrees of available tunability. Two of the most prominent approaches for 3D patterning currently being investigated are interference lithography (IL)1-7 and 3D direct write (3DDW)8-17. Superscript numbers refer to the references listed herein. The contents of all of these references are incorporated herein by reference. In the aforementioned classification scheme, IL techniques are large area, providing highly uniform structures over areas up to the cm scale by the generation of patterns either by multibeam or phase mask interference, but with little direct control over local defects. On the other hand, 3DDW provides nearly arbitrary control over 3D structures by either single photon8, multiphoton lithography (MPLDW)9-14, or other non-linear effects15-17 with subwavelength resolution (λ/4 to λ/20 with new STED inspired techniques12, 13), but at the cost of slow, serial patterning. To optimize the advantages of each technique to design scalable 3D processes for locally defined complex structures, a combined approach is a promising strategy.
Some combinations of alternative lithographic techniques have already been successfully demonstrated. Some notable examples are the utilization of DW or UV photomasks to introduce purposeful defects and create local hierarchical structures in 3D media patterned either by IL or colloidal/block copolymer (BCP) self-assembly18-21. Another technique that can be usefully combined with IL or self-assembly is multiscale nanoimprint, in which phase mask proximity field nanopatterning (PnP) IL or BCP self-assembly is performed on a structure that has already been defined at the microscale by a larger imprint step4, 22. The locally defined hierarchical structures made by these techniques are, however, limited to a uniform microstructure with the same symmetry and filling fraction as the initial 3D template.
Laser spike annealing (LSA) is an alternative to standard thermal treatment in semiconductor technology23-25. In this technique, a high intensity continuous wave (CW) or pulsed laser is rapidly scanned across an absorbing surface, such as a silicon wafer. As semiconductor materials generally possess high thermal conductivities, the local temperature at the laser spot spikes to a high value and then, once the laser light is removed, very rapidly drops back to ambient temperature. Because of this, both the temperature and annealing time can be controlled by selection of laser intensity and exposure time. Additionally, annealing can be performed while kinetically avoiding unwanted effects, such as diffusion of the gates. More recently LSA has been applied to the annealing of soft materials for the phase separation of BCPs26 and chemically amplified photoresist (CAR) post-baking27. CARs are a family of resists in which the crosslinking reaction is catalyzed by an additional photoinitiator, such as a photoacid generator (PAG) in the case of cationic crosslinking28. Photoinitiation of a single PAG creates an acid that catalyzes many crosslinking reactions. This process, in the standard CAR procedure, is accelerated by post-exposure bake (PEB), generally on a hotplate or in an oven. LSA was approached as a method to activate sub-millisecond crosslinking of a conventional CAR, thereby reducing the unwanted diffusion of photoacid into unexposed regions that occurs to a greater extent during a hotplate PEB27. This is possible due to differences in the activation energies of the two processes. The LSA of CARs was only reported for thin films of resist (˜100 nm), patterned by conventional lithography. While only nominal improvements in lithographic contrast were demonstrated, this study did show that rapid, high-temperature crosslinking of CARs on the order of 500 μs is possible. Furthermore, the quality of the resultant structures, as measured by both feature resolution and distortion due to PAG diffusion, was equal or better to conventional post-baking.